On August 3, 2011, about 0950 Alaska daylight time, a deHavilland DHC-2 airplane, N723DR, sustained substantial damage during an off-airport landing approximately 40 miles south of Northway, Alaska. The airplane was registered to and operated by Copper Valley Air Service, LLC, Glennallen, Alaska, under the provisions of 14 Code of Federal Regulations (CFR) Part 135 as an on-demand air taxi flight, under contract to the U.S. National Parks Service (NPS). The pilot and four passengers were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The public use flight originated at the Gulkana Airport, Glennallen, Alaska about 0830. The purpose of the flight was to transport NPS employees to a remote site to take water samples and do a site survey. In a written statement to the National Transportation Safety Board (NTSB), the pilot stated that during the landing, as the airplane was coming to a stop, he noticed a problem with the tailwheel. After he unloaded the passengers, he realized that the tailwheel casting had broken, and the tailwheel spindle and fork had separated from the airframe. He inspected the landing area, and noticed that he had hit a small, 4-inch, bank of dirt in the grass that he had not seen. A postaccident examination of the airplane by a Federal Aviation Administration (FAA) airworthiness inspector discovered substantial damage the aft fuselage bulkhead.